Mortgage rates dropped convincingly today, bringing them to new long-term lows. The average lender hasn't offered anything lower for more than a year (January 2018). The improvement came on a combination of news headlines, economic data, and the scheduled sale of US 10yr Treasury debt. [30YR FIXED - 4.375% - 4.5%] emphasis added

The graph shows the relationship between the monthly 10 year Treasury Yield and 30 year mortgage rates from the Freddie Mac survey.

Currently the 10 year Treasury yield is at 2.62%, and 30 year mortgage rates were at 4.41% according to the Freddie Mac survey last week.

To fall to 4% on the Freddie Mac survey, and based on the historical relationship, the Ten Year yield would have to fall to around 2.1%

To increase to 5% (on the Freddie Mac survey), based on the historical relationship, the Ten Year yield would have to increase to about 3.3%.